This application is based on Japanese application Nos. 2000-39521, 2000-152504 and 2000-154634 filed in Japan, the contents of which are hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a method of driving a liquid crystal display, and more specifically to a method of driving a liquid crystal display which comprises liquid crystal which exhibits a cholesteric phase and is capable of keeping an image displayed thereon after turn-off of the electric field applied thereto.
2. Description of Prior Art
In recent years, reflective type liquid crystal displays which use liquid crystal which exhibits a cholesteric phase at room temperature such as chiral nematic liquid crystal have been developed to be used as media for reproducing digital information as visual information because such liquid crystal displays consume little electric power and can be fabricated at low cost. However, such a liquid crystal display which uses liquid crystal with a memory effect has a demerit in that the driving speed is low.
A well-known prior art U.S. Pat. No. 5,748,277 discloses a method of driving bistable liquid crystal. The method comprises a preparation phase to cause the liquid crystal to come to a homeotropic state, a selection phase to select the liquid crystal to come to a focal-conic state or a planar state and an evolution phase to cause the liquid crystal to evolve to the desired final state. In the selection phase, by controlling the voltage applied to the liquid crystal to either one of two (high and low) levels, the state of the liquid crystal is selected.
However, this driving method has the following drawbacks: this driving method permits only a display with two tones and does not make a display with intermediate tones; this driving method requires a driving IC which has at least seven output levels to drive the scan electrodes and a driving IC which has at least two output levels to drive the data electrodes, that is, it requires costly drivers; and after the selection to determine the final states of pixels, pulse voltages are applied to the pixels from the data electrodes directly, and the image will be degraded because of crosstalk.
Incidentally, it is known that the response characteristic of chiral nematic liquid crystal to the voltage applied thereto changes depending on the temperature. Accordingly, when the temperature changes, the liquid crystal may make an incomplete display and may not be able to make a display.
When liquid crystal display layers for R, G and B are laminated to make a full-color display, the response characteristics of the respective liquid crystal materials in the liquid crystal display layers to the electric field applied thereto are different from each other. If mutually different optimal lengths are set as the respective selection steps of the display layers, the display layers will have different scanning times. The scanning time means the time from the start of selection step of a scan electrode to the start of selection step of the next scan electrode.
An object of the present invention is to provide an improved method of driving a liquid crystal display and an improved liquid crystal display device which do not have the above problems.
In order to attain the object, a first driving method according to the present invention is a method for driving a liquid crystal display which comprises liquid crystal which exhibits a cholesteric phase and is capable of keeping an image displayed thereon after turn-off of an electric field applied thereto, and the method comprises: a reset step of applying a reset pulse to make the liquid crystal come to a homeotropic state; a selection step of, after the reset step, selecting a final state of the liquid crystal; and an evolution step of, after the selection step, applying an evolution pulse to stabilize the liquid crystal to the state selected in the selection step. The selection step comprises: a first step which comprises a time of applying substantially 0 volt to the liquid crystal; a second step of, after the first step, applying a selection pulse to select the final state of the liquid crystal; and a third step, after the second step, which comprises a time of applying substantially 0 volt to the liquid crystal.
In this first driving method, since the liquid crystal is driven in the reset step, in the selection step and in the evolution step, a desired image can be written on the liquid crystal at a relatively high speed. Moreover, in the first step and in the third step of the selection step, there are times in which the voltage applied to the liquid crystal is made substantially zero and consequently, the number of output levels of a driver can be reduced.
In the first driving method, the lengths of the reset step, the selection step and the evolution step may be multiples of the shortest one of these lengths. The selection pulse may be of a lower voltage than the reset pulse.
The state of the liquid crystal may be selected by changing the pulse width of the selection pulse. Variations of the selection pulse in pulse width result in display of various intermediate tones. In this case, in the second step, the pulse width of the selection pulse is adjusted in accordance with the desired final state of the liquid crystal.
In the first driving method, the liquid crystal display further comprises a plurality of scan electrodes and a plurality of data electrodes which are arranged to face and cross each other with the liquid crystal in-between, and these scan electrodes and data electrodes define a plurality of display units of the liquid crystal display. The scan electrodes may be selected serially to be subjected to at least the selection step and the evolution step. All the scan electrodes may be selected at one time to be subjected to the reset step.
In the case of selecting all the scan electrodes simultaneously for the reset step, while the display units on one of the scan electrodes are in the selection step, an evolution pulse of a voltage with an absolute value larger than 0 is applied to the scan electrode which is to come to the selection step next. With the application of the evolution pulse, the display units on the scan electrode which is to come to the selection step next can be kept in the reset state.
In the case of selecting the scan electrodes serially for the reset step, the scan electrodes are selected at lags of a time which corresponds to the length of the selection step. In this case, at least one of the first step and the third step may include a time of applying a compensation pulse to avoid necessity of changing the length of the selection step. Thereby, the length of the selection step can be fixed in any case. If compensation pulses are applied both in the first step and in the third step, the application times of the compensation pulses in the first step and in the third step are preferably equal to each other.
At least one of the first step and the third step may include a time of applying a compensation pulse to compensate for a change in responsiveness of the liquid crystal with a change in temperature. In this case, the form of the compensation pulse depends on the temperature. Thereby, regardless of the temperature, display of a quality image can be realized at all times. Also, it is possible to change the length of the selection step depending on in what range the temperature is.
At least one of the first step and the third step may include a time of applying a compensation pulse to avoid necessity of changing the length of the selection step. In a liquid crystal display which comprises a plurality of liquid crystal layers, by applying appropriate compensation pulses to the respective liquid crystal layers, the differences among the liquid crystal layers in responsiveness can be corrected, and it becomes possible that the liquid crystal layers can have the same scanning speed. Further, the application times of the compensation pulses applied to the respective liquid crystal layers may be so set that the evolution pulses applied to the respective liquid crystal layers will have the same voltage or the same pulse width. Thereby, the structure of the power source circuit can be simplified.
In the case of selecting the scan electrodes serially, the scan electrodes are selected at lags of a specified time. In this case, the time length of the reset step is a multiple of the specific time, and the reset pulse may include a plurality of periods each of which is equal to the specific time. Preferably, in consecutive periods, the last reset pulse element in the former period and the first reset pulse element in the latter period are of the same polarity. Also, the time length of the evolution step is a multiple of the specific time, and the evolution pulse may include a plurality of periods each of which is equal to the specific time. In this case, preferably, in consecutive periods, the last evolution pulse element in the former period and the first evolution pulse element in the latter period are of the same polarity.
In other words, polarity inversion is not necessary between the last reset pulse element in the former period and the first reset pulse element in the latter period or between the last evolution pulse element in the former period and the first evolution pulse element in the latter period. Thereby, the number of polarity inversions in the reset step or in the evolution step can be reduced, and the power consumption can be reduced.
In the case of adjusting the pulse width of the selection pulse in accordance with the desired final state of the liquid crystal in the second step, the second step includes a first period of applying a pre-selection pulse and a second period of applying a post-selection pulse, and preferably, the pre-selection pulse and the post-selection pulse are of mutually opposite polarities.
The pulse width of the pre-selection pulse may be adjusted based on the beginning of the first period, and the pulse width of the post-selection pulse may be adjusted based on the beginning of the second period. Also, the pulse width of the pre-selection pulse may be adjusted based on the end of the first period, and the pulse width of the post-selection pulse may be adjusted based on the end of the second period.
A second driving method according to the present invention is a method for driving a liquid crystal display which comprises liquid crystal which exhibits a cholesteric phase and is capable of keeping an image displayed thereon after turn-off of an electric field applied thereto, and a plurality of scan electrodes and a plurality of data electrodes which are arranged to face and cross each other with the liquid crystal in-between, the scan electrode and the data electrodes defining a plurality of display units, said method comprising the steps of: applying a first voltage signal to one of the scan electrodes which defines display units which are to continue displaying an image; and applying a second voltage signal in accordance with image data to the data electrodes so that crosstalk pulses are applied to the display units which are to continue displaying an image. The first voltage signal and the second voltage signal have such waveforms that the condition that the crosstalk pulses are smaller than the second voltage signal in energy and/or pulse width can be fulfilled.
In the second driving method, the energy and/or the pulse width of the crosstalk pulses are/is small, which means that each pixel is not so strongly influenced by crosstalk from the adjacent pixels, and degradation of the image can be prevented.
A liquid crystal display device according to the present invention comprises: a liquid crystal display comprising liquid crystal which exhibits a cholesteric phase and is capable of keeping an image displayed thereon after turn-off of an electric field applied thereto; and a plurality of scan electrodes and a plurality of data electrodes which are arranged to face and cross each other with the liquid crystal in-between. The liquid crystal display device further comprises a scan electrode driver which is connected to the scan electrodes and which is capable of outputting voltages at not more than three different levels; and a data electrode driver which is connected to the data electrodes and which is capable of outputting voltages at not more than two different levels.
In the liquid crystal display device, drivers which have a small number of output levels are used, and the driving circuit is simple.